Specifications

LIMITING ELECTROMAGNETIC INTERFERENCE (EMI)

Switched mode power supplies ( SMPS ) employ high frequency switching

and thus, are a source of radio interference, a recipient of radio interference

and a conduit of radio interference. ( Older linear type transformer based

power supplies do not employ high frequency switching voltages and will be

quieter as compared to switching type of supplies ).

2. The primary emission sources originate in the switching devices due to their

fast switching current transitions: harmonics of the switching frequency and

broadband noise created by under-damped oscillations in the switching cir-

cuit. The secondary source is from the bridge rectifier, both rectifier noise

and diode recovery. The AC input rectifier / capacitor in the front end of the

switching power supplies ( excepting those with power factor correction ) are

notorious for generating power supply harmonics due to the non linear input

current waveform. The noise is both conducted and radiated through the

input power cord and the DC output wiring to the radio. Filters are used to

limit the noise to acceptable level.

3. Switching power supplies are also recipients of radio interference. The nor-

mal operation of the power supply can be disturbed due to RF noise getting

coupled into the power supply. Thus, the power supply may generate exces-

sive RF noise and lose output voltage regulation due to excessive transmitter

energy being coupled through the AC / DC lines to the power supply’s regu-

lator feedback path. This may be due to antenna being too close or due to

the antenna or feed system not radiating properly. First check the antenna

system SWR. Then, if necessary, relocate either the antenna or the power

supply farther apart.

4. The receiver may “hear” the power supply. A slowly moving, slightly buzzing

carrier heard in the receiver may be caused by the antenna being too close.

As with the transmitter related noise pick up, a loose coaxial connector or a

broken or a missing ground may aggravate this problem. Normally these

noises will be below the background or “band” noise. Increase the separation

between the power supply and the receiving antenna. Use an outdoor an-

tenna. This will reduce the amount of signal picked up from the power supply

and also increase the amount of the desired signal.

13.